A smaller hepatocyte growth factor (HGF) isoform encoding only the amino terminal and first kringle domains (HGF/NK1) has been identified and characterized. The relative levels of expression of HGF isoforms was observed to vary. depending on the cell or tissue type. Like HGF/NK2. this isoform is non-mitogenic, but binds to and activates HGF receptor autophosphorylation. The interaction of c-met with these smaller HGF isoforms are extremely unusual examples of receptor autophosphorylation dissociated from mitogenic signal transduction, and represent a unique opportunity to investigate separately these two fundamental receptor functions. To this end, isoforms were expressed in bacteria and successfully refolded and purified in large quantities of biologically active protein. Studies are now in progress to compare the effects of HGF. HGF/NK2 and HGF/NK1 binding on (1) the sites of receptor autophosphorylation. (2) the formation of receptor dimers and/or oligomers. (3) the physical association of receptor with putative second messenger molecules. (4) the intracellular fate of ligand-receptor complexes and (5) the phosphorylation of unidentified intracellular molecules. As part of the continuing effort to identify new targets and biological effects of HGF, it was observed that HGF activates integrins in distinct subsets of T-lymphocytes, thus acting as a chemotactic and proadhesive factor that may be involved in the adhesion and migration of T-cell across the vascular endothelium. Immediate HGF-induced changes in actin polymerization and cytoskeletal arrangement in the subsets of biologically responsive T-cells have been documented. It was also observed that brief HGF treatment of T-cells induced phosphorylation of several cellular proteins on tyrosine. Interestingly. Northern analysis and reverse transcriptase-polymerase chain reaction (RT-PCR) failed to demonstrate any c-met transcript in these cells. Studies are now in progress to identify the cell surface receptor that mediates these dramatic HGF effects on human T cells.